Star of the Month - January 2018

The arctic temperatures and persistent cloud cover of late have perhaps turned my mind of late to more indoor pursuits, but I have hopes for conditions near the end of this month. So it's time to think about which star gets the title role for January before it's too late.  And the winner is...

Eta (η) 24 Cassiopeiae.

Discovered by Sir William Herschel in 1779, Eta Cassiopeiae is one of the loveliest of the wintertime double stars, and I always take a moment or two to take it in when Cassiopeia is high in the sky. Stellar color is notoriously hard to pin down, and no two observers ever seem to see the same thing. I personally see two red stars of unequal brightness when viewing this one, but most observers report the brighter component as being either yellow or gold, with the dimmer companion appearing as anything from orange, red, or even purple.

Eta Cassiopeiae is about 19 and a half light years from our Solar System, and has a combined apparent magnitude of 3.44. The dimmer B component goes round the more massive primary in a highly elongated orbit, closing to 36 AU (for comparison, Neptune orbits our Sun at 30 AU) before swinging out to 106 AU at its furthest point. A single orbit takes 480 years, so we are still less than halfway through a single time round since Herschel first laid eyes on this star. Both components are extremely metal poor, indicating that they may belong to a previous generation of stars than our Sun.

Don't miss this stellar jewel, worthy of the queen which it graces. The image above was the best I could find, but it does not do justice to the reality.